FINAL_ND THRO_MP 24.1 & 24.5 Rpt_20211030.pdf

SUBMITTED TO:

HDR Engineering, Inc.

1670 Broadway, Suite 3400 Denver, Colorado 80202

BY:

Shannon & Wilson 1321 Bannock St., Suite 200 Denver, CO 80204

(303) 825-3800 www.shannonwilson.com

GEOTECHNICAL DESIGN REPORT

ND FTNP THRO 11(10)

South Unit Scenic Loop - MP 24.1 & 24.5

BILLINGS COUNTY, NORTH DAKOTA

October 2021

Shannon & Wilson No: 104347-001

Geotechnical Design Report

104347-001 October 2021 10/30/2021-104347-001_R1.docx i

Submitted To: HDR Engineering, Inc.

1670 Broadway, Suite 3400 Denver, Colorado 80202 Attn: Mr. Nick Zoller

Subject: GEOTECHNICAL DESIGN REPORT, ND FTNP THRO 11(10)

SOUTH UNIT SCENIC LOOP - MP 24.1 & 24.5, BILLINGS COUNTY,

NORTH DAKOTA

Shannon & Wilson prepared this report and participated in this project as a subconsultant to HDR Engineering, Inc. Our scope of services was specified in Agreement Number 016/1000100047399 with HDR Engineering, Inc. dated December 10, 2019 and task order modification dated October 26, 2020. This report presents our geotechnical design recommendations and was prepared by the undersigned.

We appreciate the opportunity to be of service to you on this project. If you have questions concerning this report, or we may be of further service, please contact us.

Sincerely, SHANNON & WILSON

Joseph C. Goode III Senior Geotechnical Staff

David A. Varathungarajan, PE Senior Associate

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CONTENTS

1 Introduction

2 Project Description

2.1 MP 24.1 Landslide

2.2 MP 24.5 Landslide

3 Regional Geology

4 Field Reconnaissance and Site Description

4.1 MP 24.1 Landslide

4.2 MP 24.5 Landslide

5 Subsurface Explorations and Instrumentation

6 Subsurface Conditions

6.1 Fill

6.2 Colluvium

6.3 Landslide Debris and Displaced Bedrock

6.4 Bedrock

6.5 Groundwater

7 MP 24.1 Slope Stability Analysis and Mitigation

7.1 Analysis Cross-Sections

7.2 Stability Analysis

7.3 Slip Surface Geometry

7.4 Groundwater Conditions

7.5 Material Properties

7.6 Factors of Safety

7.7 Stabilization Buttress

7.8 Additional Considerations

8 MP 24.5 Slope Stability Analysis and Mitigation

8.1 Analysis Cross-Sections

8.2 Stability Analysis

8.3 Slip Surface Geometry

8.4 Groundwater Conditions

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8.5 Material Properties

8.6 Structural Properties

Drilled Shafts

Ground Anchors

8.7 Numerical Modeling Procedure

8.8 Results

Drilled Shafts

Ground Anchors and Bond Zone

8.9 Wall Facing and Lateral Earth Pressures

8.10 Additional Considerations

9 Construction Considerations

9.1 Special Considerations

9.2 Culvert Placement

9.3 Stabilization Buttress Preparation

Clearing, Stripping, and Grubbing

Subgrade Preparation, Drains, and Benching

Compacted Buttress Fill

Slope Protection

9.4 Corrosion Potential

9.5 Drilled Shaft Considerations

Drilled Shaft Installation Methods and Equipment

Drilled Shaft Inspection and Observations

Concrete Placement

Cross-Hole Sonic Logging

9.6 Ground Anchor Considerations

10 Limitations

11 References iv

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Exhibits Exhibit 2-1: View of MP 24.1 Landslide head scarp from roadway, April, 20, 2020 Exhibit 2-2: View of MP 24.5 Landslide scarp from roadway, May 27, 2020 Exhibit 3-1: Hillshade LiDAR, MP 24.1 Landslide Exhibit 3-2: Hillshade LiDAR, MP 24.5 Landslide Exhibit 4-1: View to the south of MP 24.1 Landslide, April 20, 2020 Exhibit 4-2: View to the northwest of MP 24.5 Landslide, October 14, 2020 Exhibit 6-1: Table of groundwater levels during drilling Exhibit 8-1: Recommended Lateral Earth Pressure Design Parameters

Figures Figure 1: Vicinity Map Figure 2: Site and Exploration Plan, MP 24.1 Figure 3: Site and Exploration Plan, MP 24.5 Figure 4: Generalized Subsurface Section A-A', Station 234+37 Figure 5: Generalized Subsurface Section B-B', Station 209+43 Figure 6: Shear and Moment Diagrams, 3-Foot Diameter Shafts with Ground Anchors Figure 7: Recommended Surcharge Loading for Temporary and Permanent Walls

Appendices Appendix A: Subsurface Explorations Appendix B: Laboratory Test Results Appendix C: Reconnaissance Photographs Appendix D: Instrumentation Appendix E: Analysis Files Appendix F: Additional Corridor Data Important Information

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1 INTRODUCTION

This report summarizes the results of our subsurface exploration and laboratory testing program and presents geotechnical design recommendations and construction considerations for the South Unit Scenic Loop Milepost (MP) 24.1 and MP 24.5 Landslides (the Project) in Billings County, North Dakota (Figure 1). Our conclusions and recommendations in this report are based on:

The limitations of our approved scope, schedule, and budget described in our contract;

Our understanding of the Project and information provided by HDR;

Our observations during site reconnaissance;

Subsurface conditions observed in the borings at the time our exploration program was completed; and

The results of testing performed on selected samples collected from the exploration program.

The objective of our geotechnical studies was to provide recommendations and construction considerations, as presented herein, for stabilization of landslides at MP 24.1 and MP 24.5.

Subsurface explorations and laboratory testing were also completed at additional locations along the project corridor to provide geotechnical data to support future evaluation of potential landslides (see Appendix F). This report does not include interpretation of the data presented in Appendix F. The authorized scope of services was based on this objective and this report should not be used for other purposes without Shannon & Wilson’s review.

If a service is not specifically indicated in this report, do not assume that it was performed.

Pavement design recommendations and design for additional slope sites was not included in this scope of work.

2 PROJECT DESCRIPTION

The Project is located in Billings County, North Dakota, near Medora, within the South Unit of Theodore Roosevelt National Park (THRO). The South Unit of THRO is located along Interstate 94, with the South Unit Scenic Loop Drive providing visitors access to multiple scenic overlooks and trailheads throughout the park. Scenic Loop Drive is a two-lane roadway winding through badland terrain.

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2.1 MP 24.1 Landslide

This landslide location is also named 'Landslide Area A' and 'Main Landslide' throughout project scoping documents. Significant slope movement of the roadway at MP 24.1 of the Scenic Loop Drive (the site) occurred in May 2019, which resulted in the closure of the road.

Additional movement was noted in February 2020 during field explorations and in March and April of 2020 during instrumentation readings. The MP 24.1 Landslide includes approximately 100 feet of failed roadway, with an approximately 6- to 10-foot high head scarp. Cracking in the head scarp, which is indicative of additional slope movement after the initial failure, has been observed during our subsequent field visits.

Exhibit 2-1: View of MP 24.1 Landslide head scarp from roadway, April, 20, 2020.

The landslide toes out near the bottom of a drainage area and has exhibited ongoing movement during periodic site visits. A relatively steep bedrock slope rises above the opposite side of the drainage area. Observations of the adjacent bedrock outcrops show nearly horizontal sedimentary bedrock of the Sentinel Butte Formation. An approximately 140-foot high, north-facing, highly vegetated slope is located on the uphill side of the roadway at the landslide area. The extents of the landslide are shown on Figure 2. The

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104347-001 October 2021 landslide is described below in more detail based on aerial imagery and field reconnaissance.

2.2 MP 24.5 Landslide

This landslide location is also named 'Landslide Area C' throughout project scoping documents. This landslide mass starts within the road and continues to the downhill side of the road. Pavement cracking at MP 24.5 of the Scenic Loop Drive has been noted at the site for many years. Pavement cracks on the order of 2 to 3 inches were noted at MP 24.5 during our site visit on November 14, 2019. Additional movement has been recorded at each of our subsequent site visits, with the most notable movement occurring between March 16, 2020 and April 20, 2020. The MP 24.5 Landslide includes approximately 145 feet of significant cracking length within the roadway.

Exhibit 2-2: View of MP 24.5 Landslide scarp from roadway, May 27, 2020.

The MP 24.5 Landslide is located on the inside and downhill side of a horizontal curve. The slope is north to northeast-facing and highly vegetated. The extents of the landslide are shown on Figure 2.

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3 REGIONAL GEOLOGY

Our understanding of the site geology and subsurface conditions is based on a review of a published geologic maps, previous work completed by us in the same geologic setting, and our review of samples recovered from the borings. The project is located within the Fryburg NW, North Dakota Quadrangle, for which there is a 1:24,000 map published by the North Dakota Geologic Survey (Gonzalez, 2003). The project area is mapped as Sentinel Butte Formation, which is Tertiary in age and consists of interbedded claystone, siltstone, sandstone, mudstone, carbonaceous shale, and lignite (Carlson, 1985). Calcite cemented sandstone concretions, siderite nodules, and petrified wood are common (Gonzalez, 2003).

Exhibit 3-1: Hillshade LiDAR, MP 24.1 Landslide.

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Although the site at MP 24.1 is not described as a landslide deposit by Gonzalez, (2003), several large landslide deposits are mapped in the general vicinity. This includes a large landslide complex that encompasses the MP 24.5 active landslide. The landslides mapped by Gonzalez (2003) are described by as variable mixture of strata and deposits that have slid or slumped to the base of steep slopes. The ground surface of landslide deposits is characterized by hummocky topography, numerous arcuate scarps, and chaotic bedding.

Refer to Exhibit 3-1 and 3-2 for hillshade lidar views (Woolpert, 2020) of the MP 24.1 and MP 24.5 landslides.

Exhibit 3-2: Hillshade LiDAR, MP 24.5 Landslide.

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4 FIELD RECONNAISSANCE AND SITE DESCRIPTION

On February 21, 2020, Daniel Markowski (Shannon & Wilson engineering geologist) visited the THRO, South Unit Visitors Center where he met with Mr. Chad Sexton (Management Assistant and GIS Analyst) and Mr. Grant Geis (Chief Ranger) to discuss project details and to obtain a key to access the gate beyond the closed road. Following the discussions at the Visitor’s Center, Mr. Markowski was escorted by Mr. Sexton to the sites. An additional full corridor reconnaissance was completed in October, 2020 by Daniel Markowski and August Ridde. Refer to Shannon & Wilson's Unstable Slope Assessment Report, ND FTNP THRO 11(10), South Unit Scenic Loop - Full Corridor dated January 2020. Key observations from our reconnaissance trips are discussed in Sections 4.1 and 4.2. Additional photos from the site reconnaissance are provided in Appendix C.

4.1 MP 24.1 Landslide

The MP 24.1 Landslide is approximately 125 feet wide as measured from the maximum extents of the visible lateral flanks and approximately 100 feet wide where it directly affects the roadway and embankment to the north. The main head scarp has a significant surface expression at both the crown and lateral flanks, which has damaged both lanes of the road.

Near vertical drops of 6 to 10 feet were measured in the road at the head scarp location with exposed geogrid. A drop inlet that collects surface water runoff is still intact and drains into a corrugated steel pipe culvert which has sheared off and is suspended outside of the head scarp. There is evidence of disturbed ground extending to the toe of the slope. Several transverse tension cracks and set-down scarps measuring up to 10 feet deep were observed throughout the main body of the landslide mass.

Geotechnical Design Report

Exhibit 4-1: View to the south of MP 24.1 Landslide, April 20, 2020.

The cut slope on the uphill side of the roadway exposes intact soil and bedrock.

Observations of the exposure show near horizonal stratigraphic layers, which suggests that the slope above the road has not moved as part of a larger mass movement in the past.

Hummocky ground both west and east of the active landslide mass suggests past slope movement.

4.2 MP 24.5 Landslide

The MP 24.5 Landslide is approximately 145 feet wide where it directly affects the roadway.

Tension cracks with visible vertical offset of up to 2 feet have formed in the both lanes of the roadway. North-northeast of the existing roadway in the direction of landslide movement, hummocky ground persists to about 300 feet downslope of the roadway. Lateral flanks of the landslide narrow considerably in the direction of movement, where we interpreted a poorly defined toe bulge about 215 feet downslope.

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Other features observed downslope of the road include hydrophytic vegetation, ponded water, multiple toe bulges, and sinkholes. West of the road, uphill of the failure, we observed an outcrop of near horizonal stratigraphic layers that we interpreted to be undisturbed.

We interpret the MP 24.5 Landslide as a section of a larger, previously mapped complex that coalesces from smaller localized active landslides moving at different depths and rates.

Additional scarps and tension cracks were observed around the MP 24.5 Landslide.

However, these scarps are not currently affecting the roadway.

Exhibit 4-2: View to the northwest of MP 24.5 Landslide, October 14, 2020.

5 SUBSURFACE EXPLORATIONS AND

INSTRUMENTATION

Shannon & Wilson completed a field exploration program in February and May 2020. The field exploration included two borings at the MP 24.1 Landslide, designated SW-A-01 and

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SW-A-02 and four borings at the MP 24.5 Landslide, designated SW-C-01 through SW-C-04.

Additional explorations were also completed to provide data along the project corridor and are presented in Appendix F. Interpretation of these data is not included in this report. At the completion of drilling, all borings were completed with a bentonite-cement grouted inclinometer casing to monitor potential slope movement. A nested vibrating wire piezometer (VWP) was installed in borings SW-A-01 and SW-C-01 to measure groundwater levels. The locations of the borings are shown in Figures 2 and 3. The boring logs, additional discussion of procedures used to install instrumentation, and instrumentation data are provided in Appendices A and D.

Soil/rock samples obtained from the explorations were transported to the Shannon & Wilson laboratory in Denver, Colorado for review and testing. We completed laboratory testing of selected soil/rock samples that included natural moisture contents, unit weight, Atterberg limits, grain-size distribution and hydrometer analyses, and electro-chemical (corrosion) properties. Results from the laboratory testing program are summarized in Appendix B.

6 SUBSURFACE CONDITIONS

We encountered five geologic units in the borings: fill, colluvium, landslide debris, displaced bedrock, and bedrock (Sentinel Butte Formation). The boring logs are included in Appendix A and generalized subsurface profiles are provided in Figures 4 and 5.

6.1 Fill

Fill encountered at the MP 24.1 Landslide consisted of medium dense, poorly graded sand with silt and gravel. The fill extended to a depth of 4 feet. Geotextile fragments were encountered in the boring and also observed in the headscarp of the landslide. Fill containing clinker deposits (material baked by heat of a coal seam fire) was also observed in the headscarp at the site and consisted of red, gravel sized fragments of baked claystone, siltstone, and sandstone. The existing pavement section consisted of 5 inches of asphalt over 9 inches of aggregate base course.

Fill at the MP 24.5 Landslide consisted of very soft to stiff, lean clay to fat clay with varying amounts of sand. The fill extended to depths of 7 to 12 feet below the ground surface (bgs).

The pavement section consisted of 16.5 inches of asphalt over 9 inches of aggregate base course at boring SW-C-01, 5 inches of asphalt over 2.8 feet of aggregate base course at boring SW-C-02, and 8 inches of asphalt over 6 inches of aggregate base course at boring SW-C-03.

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6.2 Colluvium

Colluvium consists of soil that was deposited by downslope movement by either sheetwash, rockfall, or landslide activity. The colluvium at MP 24.1 was described as medium stiff to stiff, fat clay with trace sand. The colluvium extended to a depth of 12.0 feet in boring SW- A-01. Colluvium was not encountered at site the MP 24.5 site.

6.3 Landslide Debris and Displaced Bedrock

The landslide debris consists of soil or bedrock, respectively, that has moved at some point due to landslide activity. Observable characteristics of landslide debris in the soil samples include disturbed textures and tilted laminations. However, the presence or absence of these features do not suggest whether the material is actively moving. Displaced bedrock at MP 24.1 and MP 24.5 is poorly lithified and/or disturbed/weakened from movement. It is weak enough to be drilled with auger drilling equipment and mud rotary techniques, and sampled with split-spoon, modified California barrel, and thin-walled (Shelby tube) samplers. For purposes of engineering behavior, the unit is described using a dual classification (rock and soil).

At MP 24.1, landslide debris consisted of soft to stiff, fat and lean clay with varying amounts of sand. Some areas included fragments of disturbed bedrock and coal. The landslide debris at MP 24.1 is underlain by green-gray, blocky, displaced claystone from about elevation 2,601 to 2,599 feet in boring SW-A-02. This same layer was encountered from elevation 2,604 to 2,598 feet in boring SW-A-01, however it was not displaced in that boring.

At MP 24.5, soil underlying fill was landslide debris. The landslide debris included material inside and outside the limits of the current active landslide (i.e. landslide debris related to historic or relict landslides exists outside of the current active slide limits). Landslide debris in this location was typically very soft to stiff, lean and fat clay with varying amounts of sand and to a lesser extent consisted of loose, clayey sand. Displaced bedrock was encountered in boring SW-C-03 from a depth of 22.0 to 33.0 feet (elevation 2,604 to 2593 feet) and was characterized by extremely weak, laminated to blocky, brown, gray, and dark brown fat clay with tilted laminations up to 40 degrees.

6.4 Bedrock

The bedrock at the site consisted of Sentinel Butte Formation. Our explorations encountered extremely weak claystone that varied in color from dark brown to gray to green-gray. The claystone was laminated to thinly bedded, sometimes blocky, and varied from moderately

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104347-001 October 2021 weathered to slightly weathered. We also encountered layers of extremely weak coal that was dark brown to black in color.

Based on our experience with this geologic formation, the color of the claystone can be indicative of some physical and engineering characteristics. Dark brown claystone typically indicates claystone that is near a transition to a coal seam and green-gray claystone typically indicates claystone with relatively high plasticity (liquid and plastic Atterberg limits).

Laboratory testing indicated liquid limits (LLs) for the bedrock ranging from 38 to 190, with the maximum value occurring in the green-gray layer.

6.5 Groundwater

Groundwater levels measured during drilling, upon completion of drilling, and most recent groundwater levels measured by the VWPs are indicated on the boring logs and below in Exhibit 6-1. Data obtained from the VWPs at SW-A-01 and SW-C-01 are presented in Appendix D.

Exhibit 6-1: Table of groundwater levels during drilling

Boring Designation

Groundwater Elevation

During Drilling (Date) Highest Reading (Date) 1 Most Recent Reading (Date) 1

SW-A-01 2,605.3 (02-26-2020) 2,615.2 (07-21-2020) 2,604.7 (10-7-2021)

SW-A-02 NE (05-30-2020) - -

SW-C-01 2,587.7 (02-27-2020) 2,592.1 (03-05-2020) 2,588.2 (10-7-2021)

SW-C-02 2,577.8 (05-28-2020) - -

SW-C-03 2,609.5 (05-29-2020) - -

SW-C-04 2,569.6 (05-27-2020) - -

Notes:

1. Data only available for borings installed with a VWP and data logger.

ft = feet; NE = Not Encountered

Fluctuations of groundwater levels at the site are possible and will depend on many factors, including seasonal variations and local precipitation.

7 MP 24.1 SLOPE STABILITY ANALYSIS AND MITIGATION

Based on discussions with HDR and Central Federal Lands (CFL), a buttress was selected as the preferred approach to mitigate the MP 24.1 Landslide. Other options may be feasible but were not considered further because the site topography, specifically the adjacent slope rising from the opposite side of the toe of the landslide, is conducive to a buttress repair.

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To assess the required buttress configuration, we completed analyses for representative cross-sections. Because the slope has moved significantly, the factor of safety (FS) of the slope can be considered to be 1.0 at the time of failure (Duncan and Wright, 2005).

Assuming a FS of 1.0 at failure, a stability model that is representative of conditions at the time of failure can then be developed and used as a baseline for design of the stabilizing buttress.

7.1 Analysis Cross-Sections

We completed the back-analyses at a representative cross-section based on survey provided by HDR and our projection of pre-failure ground surface at project stationing STA 234+37 (see Figure 4). Our analyses were based on our borings and field observations of the landslide.

7.2 Stability Analysis

We completed two-dimensional limit-equilibrium stability analyses using the software SLOPE/W 2019 (Geo-Slope International, Ltd., 2020). We used the Morgenstern and Price method of slices, which satisfies force and moment equilibrium, to compute FS values and drained, or long-term, stability analyses. Based on the relatively low seismic hazard for the Project site, we did not consider seismic loading.

7.3 Slip Surface Geometry

Our understanding of the slip surface geometry is based on boring SW-A-02 completed at the MP 24.1 Landslide, the head scarp, and toe bulge. One boring was completed outside of the landslide area (SW-A-01). Limited movement has been detected in inclinometer SW-A- 01 over our monitoring period (February 2020 through October 2021). It is unclear if this movement is associated with progressive landslide movement or sympathy movement associated with the near vertical adjacent headscarp. Regardless, the proposed solution presented herein will address either situation.

Boring SW-A-02, completed within the landslide area encountered displaced green-gray claystone at a depth of 14 feet (elevation 2,601.1), and the inclinometer at this location was sheared off at a depth of 12 feet prior to our second reading approximately 2 weeks after installation.

Landslide activity in sedimentary bedrock, such as the Sentinel Butte Formation found at the project location, is often controlled by near-horizontal seams of comparatively weaker material. As such, we considered a failure surface coinciding with the green-gray claystone

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104347-001 October 2021 layer identified between elevations 2,603.5 and 2,597.8 feet, which had a LL of 108 to 191.

Specifically, we considered a failure surface with an entry point corresponding to the observed scarp location and a horizontal portion coinciding with the green gray layer that daylighted at the face of the slope.

7.4 Groundwater Conditions

We initially selected a piezometric surface representative of groundwater conditions encountered in the subsurface explorations and measured in the groundwater instrumentation. We then adjusted the piezometric surface as necessary to produce a FS of

1.0 for the back-analysis.

7.5 Material Properties

We selected shear strength parameters based on correlations with plasticity and our judgement. We used published correlations for residual shear strength envelopes based on liquid limits and clay size fraction (Gamez and Stark, 2016). The parameters were selected based on the laboratory testing from completed borings and our experience in similar geologic conditions.

7.6 Factors of Safety

Based on AASHTO (2020) requirements, slopes that do not support structural elements (e.g.

bridge substructures) are typically designed for a resistance factor (RF) of 0.75. The resistance factor is essentially the inverse of the FS for slope stability. As such, we utilized a design FS of 1.3.

7.7 Stabilization Buttress

After completing the back-analysis to an FS of 1.0 with pre-slide topography, we analyzed various buttress configurations required to provide an FS of 1.3 with the current topography. Based on our analyses, a 1V:3.5H buttress fill slope provides adequate slope stability (FS of at least 1.3). The 1V:3.5H slope should begin at the shoulder of the roadway and extend down to the slope rising from the opposite side of the drainage. Because this will result in fill being placed in the existing drainage at the toe of the slope, it will be necessary to install a culvert to maintain drainage at toe of the existing slope.

We recommend that the proposed buttress extend approximately 50 feet beyond the extents of the active scarp.

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7.8 Additional Considerations

The current conditions include a steep scarp along with poor drainage, providing a water source to further destabilize the slope. Additionally, the current scarp will likely continue to ravel back into the slope if left unmitigated. Therefore, we recommend implementing the site slide repair as soon as reasonably possible to reduce the potential for progressive slope movement to extend uphill of the roadway.

8 MP 24.5 SLOPE STABILITY ANALYSIS AND MITIGATION

Based on discussions with HDR and CFL, an anchored soldier pile/drilled shaft system installed along the downhill edge of the roadway was selected as the preferred approach to mitigate the MP 24.5 Landslide. Other options may be feasible but were not considered further based on site limitations and the desire to construct a fix from the existing roadway, which will provide a solution with less invasive construction. The selected solution will protect the roadway, however overall landslide stabilization will not be accomplished. As such, movement of the ground downhill of the wall should still be anticipated. Our analyses should be considered preliminary and should be reevaluated during final design.

To assess the required drilled shaft and ground anchor configuration, we completed limit equilibrium analyses and a numerical model for a representative cross-section. We completed numerical analyses using the software FLAC Version 8.0 (Itasca, 2016), a two-dimensional, plane strain, finite difference program specifically developed to model soil-structure interaction.

8.1 Analysis Cross-Sections

We completed the back-analyses of a representative cross-sections based on survey provided by HDR at project stationing STA 209+43 (see Figure 5). Our analyses were based on the borings and field observations of the failed landslide.

8.2 Stability Analysis

We completed two-dimensional limit-equilibrium stability analyses using the methods described in Section 7.2. The results from limit-equilibrium back-analysis to a FS of 1.0 were used as a check on the FLAC model.

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8.3 Slip Surface Geometry

Our understanding of the slip surface geometry was based on our field observations, inclinometers, and borings completed from the roadway. We considered a failure surface with an entry point corresponding to the observed scarp location and the movement noted in boring SW-C-01 and SW-C-04. Due to access limitations, it was not possible to complete explorations downhill of the roadway. Therefore, we assumed a slip surface exit point corresponding to the interpreted slide toe. This produced a near horizontal failure surface, which is consistent with failures in sedimentary bedrock controlled by low-strength layers.

8.4 Groundwater Conditions

We initially selected a piezometric surface representative of groundwater conditions encountered in the subsurface explorations and measured in the groundwater instrumentation. We then adjusted the piezometric surface as necessary to produce a FS of

1.0 for the back-analysis.

8.5 Material Properties

We selected shear strength parameters based on correlations with plasticity and our judgement. We used published correlations for residual shear strength envelopes based on liquid limits and clay size fraction (Gamez and Stark, 2016). We then utilized a Mohr- Coulomb, perfectly elastic, perfectly plastic constitutive model to represent the soil/rock for FLAC. We selected elastic parameters (bulk and shear moduli) based on published correlations (Das, 1999) and our experience.

8.6 Structural Properties

Drilled Shafts

The drilled shafts were modeled using a vertical line of connected “beam” elements. Beam elements are two-dimensional elements with three degrees of freedom (translation in two directions and rotation) that can model axial and bending resistance. To simplify the model and because structural design of the drilled shafts will be completed by others, we assumed infinite axial and bending strength for the beam elements. We assigned elastic stiffness parameters corresponding to a cracked section for 3-foot diameter drilled shafts with 1.5 percent reinforcement steel. 70-foot long drilled shafts were modeled spaced at 9 feet center-to-center along the downhill (northeast) side of the existing roadway. It may be feasible to shorten the overall length of the shaft on the order of 5 to 10 feet during final design depending on final layout and loading.

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Ground Anchors

We placed the ground anchors approximately 10 feet below the top of the drilled shafts. We assumed the following for ground anchors:

Allowable tensile load of about 141 kips (which corresponds to a 4-strand ground anchor), An allowable bond strength of 2 kips per square foot (ksf) in Sentinal Butte Formation rock, A bond diameter of 6 inches, an anchor spacing of 9 feet center to center, and an inclination of 15 degrees below horizontal.

A free length of 60 feet and bond length of 40 feet, resulting in the bond zone being located below the slip surface.

These parameters correspond to an allowable bond capacity of about 126 kips.

We selected the allowable bond strength of 2 ksf, which corresponds to an ultimate bond strength of 4 ksf with an FS of 2, based on the SPT N-values, anchor load in similar geologic conditions, recommendations in FHWA Manual "Ground Anchors and Anchored Systems", GEC No. 4 (Sabatini and others, 1999), and our judgment. While GEC No. 4 suggests an FS of 2.5 on the bond strength to account for potential creep in high plasticity clay, such as the material comprising the bond zone at the site, a slightly lower FS of 2 is reasonable in our opinion based on the results of anchor testing in similar material (e.g. the Painted Canyon Landslide repair near Medora).

The contractor will be responsible for selecting appropriate means and methods for installing ground anchors that provide the assumed bond strength. The contractor will also be required to complete performance and proof tests of production anchors in accordance with the Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14 (2014) Section 256.08 to confirm the anchor bond strengths during construction. Additionally, we recommend a sacrificial verification test be completed prior to construction to confirm the bond strength that can be achieved at the site using the contractor's proposed installation methods.

8.7 Numerical Modeling Procedure

To evaluate the loads in the structural elements, the soil/rock strengths were reduced by a given strength reduction factor (SRF). The following equations were used to reduce the soil/rock strengths:

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∅𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 = tan−1 � tan∅

𝑆𝑆𝑆𝑆𝑆𝑆

𝑐𝑐𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 = 𝑐𝑐𝑟𝑟𝑢𝑢𝑢𝑢

𝑆𝑆𝑆𝑆𝑆𝑆

As the soil strengths were incrementally reduced to the target FS (SRF) of 1.3, relatively large ground deformations occurred such that mathematically impossible zone geometry (“bad geometry”) developed in some areas of the model (e.g., adjacent nodes crossed each other) and the model was unable to run to equilibrium. To allow the model to run to equilibrium for a given FS, we flattened out downslope geometry based on intermediate movement to allow the soil to displace up to 10 feet laterally along an interface.

We considered the following limit states in our analysis:

Service Limit State: we considered a SRF of 1.1 to determine service level deflections.

Strength Limit State: we considered a SRF of 1.3 to determine factored shears and moment to be used for structural design of the drilled shafts.

Extreme Limit State: the extreme limit state was considered to evaluate the possibility of a full setdown of the material in front of the wall to the existing slip surface elevation.

For this case, full ground loss was assumed on the downhill side of the wall above the slip surface. We considered a SRF of 1.1. In our opinion, it would be appropriate to design the shafts for these conditions due to the uncertainty in the slip surface geometry downhill of the roadway (it was not feasible to complete explorations in this area, nor could a well-defined toe be identified during the reconnaissance).

8.8 Results

Drilled Shafts

Design shear and moment diagrams for drilled shafts at the strength and service event limit states are shown in Figure 6. The plots are each for a single drilled shaft. The values have been factored for the given limit state and do not need to be multiplied by additional load factors.

Ground Anchors and Bond Zone

As discussed in Section 8.6, we assumed an allowable anchor load of 126 kips and a FS of 2 on the anchor bond strength based on our experience with load tests in similar material for our analysis. Based on LRFD criteria, we recommend that all ground anchors be proof tested to a maximum load equal to the factored design load, 170 kips. We determined the factored design load by multiplying the service ground anchor load (126 kips) by the

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AASHTO (2020) resistance factor specified for anchored retaining structures (1.35). Design of the cap beam should account for the maximum allowable test load for a 4-strand ground anchor, 188 kips, and the ultimate load, 234 kips. During construction, the ground anchors should be locked-off at an anchor load of 117 kips.

In addition to performance and proof testing in accordance with FP-14 Section 256.08, the bid documents should require the contractor to complete verification testing on sacrificial ground anchors to determine the ultimate bond strength. The contractor will be required to prove the assumed nominal anchor bond resistance of 252 kips.

8.9 Wall Facing and Lateral Earth Pressures

We anticipate that the wall facing will consist of either timber lagging, shotcrete, cast-in-place concrete, or precast panels. The lateral earth pressures on facing elements depend on many factors, including construction sequencing and type of facing. We assume that the facing will be relatively flexible relative to the drilled shafts and could be designed for active conditions. The active earth pressure parameters provided in Exhibit 8-1 assume the walls are backfilled with properly compacted onsite clays and have been multiplied a reduction factor of 2/3 to account for arching.

Exhibit 8-1: Recommended Lateral Earth Pressure Design Parameters

Design Parameter On-site Clay Fill -

Design Value

Active Earth Pressure Coefficient, KA 0.25

Equivalent Fluid Density for Active Conditions (pcf) 30 pcf = pounds per cubic foot; psf = pounds per square foot

Surcharge loads from motor vehicles/traffic and construction equipment will induce additional lateral loads on walls. The pressures provided in Exhibit 8-1 do not account for surcharge loads. Lateral loads due to various types of surcharges may be calculated by multiplying the surcharge by the active earth pressure coefficient (KA). The earth pressure coefficients are also shown in Exhibit 8-1. Surcharge loads should be applied as recommended on Figure 7.

8.10 Additional Considerations

We recommend that the proposed wall extend approximately 50 feet beyond the extents of the active scarp. Consideration could also be given to extending the wall beyond the extents of the relict landslide, depending on project funding and CFL's risk tolerance for future landslide movement affecting the roadway.

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The facing will extend to the construction bench used to complete ground anchor installation (the facing will not be buried). The wall has been designed to retain the soils within the roadway, however downslope movement is still possible and some setdown of the downslope is possible, which was considered in our analysis. If setdown occurs, a portion of the facing may become undermined, exposing soil between adjacent drilled shafts. If this occurs, long term maintenance may be required to install additional facing to prevent raveling of exposed soils.

9 CONSTRUCTION CONSIDERATIONS

Earthwork, including site preparation, embankment construction, placement of fill, and subgrade preparation, should conform to the requirements provided in Division 200 of the Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14 (2014) and the additional requirements presented herein.

9.1 Special Considerations

Construction of the stabilization buttress at MP 24.1will require careful monitoring of all construction activities as the existing slope is marginally stable, as indicated by recently observed cracks in the scarp area. Special considerations and precautions are necessary to limit the potential for the landslide to activate again during construction.

We recommend that all work proceed from the bottom of the slope, as such, it will be necessary for the contractor to construct an access road from the Scenic Loop Drive down to the base of the drainage. Significant cuts and fills should be avoided in preparing this access. In addition, cuts into the existing failed slope should be limited or avoided to the extent possible during culvert preparation and construction. We recommend that benches only be opened that can be backfilled within 48 hours, as the landside is marginally stable.

Further, movement of earthwork equipment should be monitored, and limited to the extent possible to complete the work.

Surficial runoff and groundwater should be expected at the project site. Any drainage features from the roadway should be diverted from the work area, including the existing corrugated metal pipe within the failed scarp. Construction timing should consider and account for the seasonal variation in precipitation.

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9.2 Culvert Placement

We recommend that the culvert placement at the base of the drainage be placed in an incremental fashion, with no more than 20 feet of the failed landslide toe removed at a time.

We recommend that trees, shrubs, surface vegetation, topsoil, and organic rich soils within the drainage be removed prior to subgrade preparation. Based on our field reconnaissance we anticipate the presence of soft, clayey landslide debris in the drainage. In addition, the failed toe may be impounding water leading to areas of potentially weak, wet subgrade soils.

The stabilization buttress will result in the placement of 10 to 15 feet of fill at the culvert location. As such, design of the culvert will need to consider the potentially weak subgrades currently observed within the drainage. We recommend a minimum of 6 inches of FP-14 704.02 Bedding Material be placed below the culvert. Any soft clays and disturbed soils encountered during subgrade preparation should be over-excavated to expose a firm and dense subgrade. Over-excavated material should be replaced with Bedding Material.

In the case that unstable subgrade is not encountered within a depth of 2 feet below the culvert invert elevation, we recommend placing a Separation and Stabilization Geotextile (FP-14 714.01) on the subgrade followed by the placement of Bedding Material.

9.3 Stabilization Buttress Preparation

Clearing, Stripping, and Grubbing

We recommend that trees, shrubs, surface vegetation, topsoil, and organic rich soils be removed in an incremental fashion as part of subgrade preparation (described in Section 9.3.2). Topsoil and organic rich soils can be re-used in landscape areas once the buttress grading is complete.

Subgrade Preparation, Drains, and Benching

From the base of the drainage, we recommend clearing, grubbing, and excavating to a depth of approximately 2 feet (matching the existing slope angle) up to approximately elevation 2,607 feet, which corresponds to two feet above the elevation of the water bearing coal seam layer encountered in boring SW-A-01. We then recommend placing a drainage blanket along the slope consisting of 24 inches of FP-14 704.07 Select Borrow wrapped in FP-14

714.01 Separation Geotextile, Class 1, Type C or E. The drainage blanket should outlet to an appropriate drainage location.

After completing the buttress backfill to elevation 2,607 feet, we recommend constructing the stabilization buttress incrementally in accordance with FP-14 Section 204, but modified

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104347-001 October 2021 with 5-foot maximum bench heights, measured from the bench elevation to the catch point on the slope. Refer to Section 9.3.3 for compacted backfill recommendations.

We recommend that springhead drains be installed to intercept and divert any seeps that are observed out of the buttress above elevation 2,607 feet during benching. Springhead drains should consist of a 4-foot minimum width and 2-foot minimum height of FP-14

704.07 Select Borrow wrapped in FP-14 714.01 Separation Geotextile Class 1, Type C or E, constructed with a minimum slope of 2% and daylight to the face of the buttress.

Compacted Buttress Fill

All fill soil placed within the stabilization buttress should be compacted Clay Fill.

Compacted Clay Fill should consist of soil that is free of organics (including lignite), contaminants, debris, and rock fragments larger than 3 inches and a minimum unit weight of 125 pounds per cubic foot. It should be noted that the locally available clay will be sensitive to small changes in moisture content, and adequate compaction will be difficult to achieve.

Clay Fill should be moisture conditioned and compacted in accordance with FP-14 Section

204. All backfill should be shaped to promote positive drainage and prevent low spots that may hold water.

Slope Protection

Upon completion of the stabilization buttress, we recommend that slopes be seeded and/or planted with relatively fast-growing vegetation to reduce the potential for surface sloughing and erosion. Natural or geosynthetic erosion control blankets may be necessary to assist in erosion protection during vegetation growth.

Any drainage outlets on the slope should be reinforced with concrete, rip-rap or similar to protect against erosion. We recommend that the existing shoulder drain be reconstructed and tight-lined to outlet at the base of the slope.

9.4 Corrosion Potential

The soil and rock encountered at the project site can be corrosive to substructure elements. To assist in estimating the corrosion potential at the site, a selected sample was tested for pH, resistivity, water soluble sulfates, and chlorides. The results are presented in Table B-1 in Appendix B.

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The resistivity measured in the sample was 212 ohm-centimeters. Based on correlations developed by Roberge (2012), this value suggests an extremely corrosive subsurface conditions for metal in contact with subsurface materials across the site.

The concentration of water-soluble sulfates measured in the sample was 0.315 percent by weight. Based on classifications as defined by ACI-318-19 (ACI, 2019), these test results suggest an exposure class S3 on concrete exposed to site soils.

The test results and the above discussion are provided to assist the designer in the selection of project materials, concrete type, or other features with respect to corrosion. As appropriate, the designer should consider protective measures, such as coatings, upsizing for section loss, or using alternative materials to reduce the corrosion potential.

9.5 Drilled Shaft Considerations

Prior to construction of the drilled shafts, we recommend removing the upper 5 feet of pavement and soil at the wall location to unload the active landslide prior to loading with construction equipment. The width of the soil removal should be approximately 40 feet, starting at the front of the proposed wall location. In addition, we recommend the contractor sequence the construction from each end of the wall and work towards the center to provide improved stability prior to working within the unstable landslide area.

Drilled Shaft Installation Methods and Equipment

Specifications and installation methods should be in general accordance with our recommendations and guidelines in the 2018 Federal Highway Administration (FHWA) manual, “Drilled Shafts: Construction Procedures and Design Methods”, GEC No. 10 (Brown and others, 2018).

Drilled shafts will extend into the claystone. Our experience indicates heavy duty drill rigs using auger drill methods can usually penetrate bedrock similar to that encountered at the site. The specifications should require the drilled shaft contractor to demonstrate experience drilling in similar bedrock formations and adequate evaluation of bedrock conditions, to confirm proposed methods and expected production.

We anticipate that temporary stabilization will be required to prevent sloughing of the sidewalls during drilling. An oversized hole could be drilled and supported by drilling slurry or placing temporary casing down to competent bedrock. Construction of drilled shafts using wet methods (i.e., slurry) is more difficult than constructing shafts using dry methods. Because a wet excavation cannot be easily visually observed, good construction practices are critical to constructing shafts that perform adequately. Wet installation

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104347-001 October 2021 methods and specifications should be in accordance with the FHWA GEC No. 10 (Brown and others, 2018).

Drilled Shaft Inspection and Observations

The hole should be cleaned of loose material and observed by a representative of the geotechnical engineer prior to pouring concrete. The drilling and concreting process should be relatively continuous with minimal stoppage of work between the completion of drilling, cleaning the hole, and the placement of concrete after setting the rebar cage.

Concrete Placement

Groundwater inflow into drilled shafts from the overburden and within fractured or more permeable zones within the bedrock is likely. Tremie concrete placement may be required if significant water inflow develops or shafts are constructed using slurry. Tremie placement should be used if wet methods are used to construct the shafts or if water cannot be controlled by pumping or bailing such that more than 3 inches of water is present when concrete is placed.

We recommend the concrete mix be designed and placed in accordance with FHWA GEC No. 10 (Brown and others, 2018). Assuming concrete will be placed with tremie methods, a minimum slump of 8 inches should be specified. When casing and/or tremie concrete placement methods are used, a minimum head of concrete of 5 feet above the bottom of the tremie pipe or casing should be maintained at all times.

Drilled shaft defects in cased shafts are frequently the result of inadequate head of concrete, particularly when combined with marginal or low slump concrete. Adequate methods should be established to measure and confirm that minimum head requirements are met throughout the concrete placement process.

Cross-Hole Sonic Logging

We recommend that Cross-Hole Sonic Logging (CSL) be used to evaluate the integrity of the drilled shafts. CSL is a non-destructive testing method that requires steel (preferred for durability and to avoid delaminating from the concrete) or plastic tubes installed in the drilled shaft and tied to the rebar cage. The tubes are attached to the interior of the rebar cage and then the cage is lowered into the hole and the concrete is placed. Typically, one access tube per foot of shaft diameter is placed inside the rebar cage. After the concrete has cured, a sound source and receiver are lowered, maintaining a consistent elevation between source and sensor. A signal generator generates a sonic pulse from the emitter which is recorded by the sensor. Relative energy, waveform, and differential time are recorded and

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104347-001 October 2021 logged. This procedure is repeated at regular intervals throughout the shaft. By comparing the graphs from the various combinations of access tubes, a qualitative idea of the soundness of the concrete throughout the drilled shaft can be interpreted.

9.6 Ground Anchor Considerations

Specifications and installation methods should be in general accordance with FP-14 Section 256 and guidelines in the FHWA Manual, “Ground Anchors and Anchored Systems", GEC No. 4 (Sabatini and others, 1999). Ground anchor testing should include the following:

A minimum of one sacrificial verification test prior to construction.